This invention relates generally to power supplies and methods and more particularly, but not by way of limitation, to a battery pack having switchable sets of batteries for powering a downhole tool during different time periods during which different currents are required so that the tool is operable for a longer overall time.
In the oil and gas industry downhole electronic equipment for recording pressure, temperature or other well data is usually powered by batteries contained in the equipment to be lowered into a well bore. One type of battery known to have been used in such an environment is a zinc silver oxide battery, or groups of these batteries, which can supply both relatively high and low currents within an operating temperature range between approximately 15.6.degree. C./60.degree. F. and approximately 170.degree. C./338.degree. F. Another known type of battery is a lithium based battery which has heretofore had somewhat limited, if any, utility in such an environment because this type of battery cannot provide relatively high currents which are sometimes needed by the downhole electronic equipment. The lithium based battery is also primarily an elevated temperature (e.g., above approximately 100.degree. C./212.degree. F.) battery so that this type might inadequately provide the relatively low currents at lower temperatures which might be encounted, such as an ambient surface temperature at the mouth of the well or other relatively low temperature at locations in the upper hole.
The lithium type battery does, however, have one particularly desirable characteristic, namely, an energy density which is significantly greater than the energy density of the zinc silver oxide type battery. For example, the energy density of a zinc silver oxide battery might be only 50% to 70% of the energy density of a lithium battery. That is, lithium batteries contain more electrical energy than comparable zinc silver oxide batteries; however, the former are useful only at relatively low currents and generally at relatively elevated temperatures. This means that, in a suitable low current drawing, high temperature environment, a lithium battery would have a longer operating life than a comparable zinc silver oxide battery under similar continuous operating conditions.
The foregoing differences in types of batteries which might be used in downhole electronic equipment are significant because such equipment generally has different current requirements during different periods of operation of the equipment. For example, a downhole electronic recorder that stores data in EPROM memory might require three different currents. For the EMR 502/504 memory gauge from Geophysical Research Corporation (GRC), the three currents required are typically 200 milliamps (mA) for 7.1 seconds to sample data, typically 500 mA for 0.1 second to store data, and typically 20 mA for up to several minutes to wait until the next data sample time. In a typical run while being powered exclusively by zinc silver oxide batteries, this tool might operate for 120 to 150 hours and record 2000 to 4000 pressure or temperature samples. A majority of the energy expended from the zinc silver oxide batteries in operating this tool would be expended at the low standby current of typically 20 mA while waiting between samples. This could be more efficiently operated by the higher energy density lithium type battery than the relatively lower energy density zinc silver oxide type battery; however, the lithium type battery cannot provide the relatively high currents needed to sample and store the data.
To more efficiently operate such equipment and thereby to extend the operating time of such equipment, there is the need for an improved power supply, and related method, which provide relatively high currents when needed, but which use a more efficient or higher energy density source to provide the relatively lower currents which are more often required. Such a power supply and method should result in increased run times for the electronic equipment, or load, energized by such power supply and method. It would also be desirable for such an improved power supply and method to be less expensive than other types which may have heretofore been used. It would also be desirable if such an improved power supply and method could produce a space saving, which is important when such a power supply and method would be used with a downhole electronic equipment which of necessity has limited space so that it will fit into the well bore.